docs: def mfds2multimfd(mfds):
      """
      Convert a list of MFD nodes into a single MultiMFD node
      """
      _, kind = mfds[0].tag.split('}')
      node = Node('multiMFD', dict(kind=kind, size=len(mfds)))
      lengths = None
      for field in mfd.multi_mfd.ASSOC[kind][1:]:
          alias = mfd.multi_mfd.ALIAS.get(field, field)
          if field in ('magnitudes', 'occurRates'):
              data = [~getattr(m, field) for m in mfds]
              lengths = [len(d) for d in data]
              data = sum(data, [])  # list of lists
          else:
              try:
                  data = [m[alias] for m in mfds]
              except KeyError:
                  if alias == 'binWidth':
                      # missing bindWidth in GR MDFs is ok
                      continue
                  else:
                      raise
          node.append(Node(field, text=collapse(data)))
          if lengths:  # this is the last field if present
              node.append(Node('lengths', text=collapse(lengths)))
      return node